SALEM TOWNSHIP (KDKA/AP) — Investigators have found evidence of corrosion on a natural gas pipeline that exploded in a massive fireball in Westmoreland County last week, scorching trees a quarter-mile away, damaging homes and burning a fleeing homeowner, the federal pipeline safety agency said Wednesday.

The cause of Friday’s blast remains unknown, but the corrosion indicates a “possible flaw” in the coating material applied to a pair of welded joints, one at the point of failure and another excavated after the explosion, the Pipeline and Hazardous Materials Safety Administration said in a corrective order issued to the pipeline operator.

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“One at the point of failure and another excavated after response to the failure site,” said Susan Waller, of Spectra Energy. “The pattern of corrosion indicates a possible flaw in the coating material applied to the girth weld.”

Houston-based Spectra Energy Corp. said a 2012 pipeline inspection did not reveal any sections that required repair. Spectra and federal officials did not address questions about the safety of other sections of the pipeline in light of the possibly defective coating.

The coating material, which is supposed to protect pipelines from corrosion, was applied when the 30-inch pipeline was installed in 1981 “following construction welding procedures in the field at the time,” the agency said.

Decades ago, that coating was an “accepted and preferred method of coating” the pipeline weld joints, Spectra spokesman Creighton Welch said in an email.

The company conducts a high-tech pipeline inspection every seven years, as well as routine field surveys of corrosion prevention systems, he added.

The explosion in Salem Township, destroyed one home, damaged three others and prompted evacuations. The owner of the leveled home, 26-year-old James Baker, is hospitalized with burns to more than 75 percent of his body.

He is being treated at UPMC Mercy Hospital in Pittsburgh and is listed in fair condition.

Kerry Jobe and other township supervisors are helping Baker’s family with fundraisers and outreach.

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“Apparently, his condition isn’t as good as we all had hoped,” said Jobe, a Salem Township supervisor. “The burns are a lot worse than everybody anticipated and he’s going to continue to have surgeries, and they’re going to have some tough decisions to make here in the future.”

The enormous blast propelled a 25-foot section of pipe about 100 feet from the epicenter and produced a crater that measured 30 feet wide, 50 feet long and 12 feet deep, according to documents released Wednesday.

The damaged pipeline and three nearby pipelines are out of service while the inquiry continues.

It’s also still unclear when busy Route 819 will reopen.

“Obviously, they’re going to repair the road, but they have to do some additional testing before they make the repairs, and we just figured that it would be safer for everybody involved down at the site if we just kept the roadway closed at this time,” said Forbes Road VFD Chief Bob Rosatti.

The pipeline safety agency ordered Spectra to inspect the pipeline in the vicinity of the blast for “corrosion, coating condition” and other damage and make repairs as necessary, and to take other measures.

Investigators are looking closely at three other pipelines to make certain this never happens again.

“This is not something we anticipate, but the investigation is ongoing,” said Waller.

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